1. Field of the Invention
The present invention relates to a technique for efficiently coating a large number of electronic component chips with metal films for providing external terminals, for example, and more particularly, it relates to an electronic component chip holder which can be advantageously employed for holding a plurality of electronic component chips for such an operation. The invention also relates to a method of using such an electronic component chip holder, and more specifically, it relates to a method of performing metal coating on metallized surfaces of electronic component chips.
Throughout the specification, the term "electronic component chips" includes the so-called partially fabricated items to be subjected to soldering for preventing oxidation of metallized surfaces for serving as external terminals, mounting of lead wires and the like, for example, in addition to finished chip-type electronic components to be directly mounted on circuit boards.
2. Description of the Background Art
An electronic component chip serving as a capacitor, a resistor or an inductor, for example, is generally provided with external terminals on both longitudinal ends. In order to form such external terminals, for example, silver paste is generally applied to prescribed positions of the chip body, and is then baked. However, such metallized surfaces containing silver are easily oxidized. Therefore, the silver-metallized surfaces are coated with solder, for example, to prevent oxidation. Such solder coating is also adapted to prevent suffering silver from the so-called solder leaching when the electronic component chip is soldered onto a circuit board by means of the external terminals. In a similar point of view, such solder coating may not be directly applied on the silver-metallized surfaces, but a tin layer may be interposed therebetween.
In order to perform the aforementioned solder coating by a conventional method, basically an electronic component chip already having surfaces metallized with silver or the like is dipped into a molten bath of solder. Thus, films of solder are selectively formed on the metallized surfaces. In mass production, a large number of extremely small-sized electronic component chips must be efficiently processed. Therefore, the electronic component chips are generally handled by means of a holder which can hold a large number of such electronic component chips.
FIG. 10 shows a first example of an electronic component chip holder, which has been employed in the plant of the assignee. This electronic component chip holder 1 is entirely in the form of a plate, which is prepared from a metal of inferior solderability such as stainless steel, for example. A pressure-sensitive adhesive double-coated tape, for example, is adhered onto the upper end surface of the holder 1, thereby to define an adhesive face 2. On the other hand, each of the electronic component chips 3 is provided with metallized surfaces 4 and 5 on both of their respective longitudinal ends. A plurality of such electronic component chips 3 are so adhered onto the adhesive face 2 that the respective metallized surfaces 4 and 5 are separated from the adhesive face 2, to be held by the holder 1.
The plurality of electronic component chips 3 held by the holder 1 as shown in FIG. 10 are dipped in a molten bath of solder (not shown). Thus, both of the metallized surfaces 4 and 5 are brought into contact with the molten bath of solder. Then, the electronic component chips 3, being held by the holder 1, are raised up from the molten bath of solder, to solidify parts of the solder adhered on the metallized surfaces 4 and 5. Thus, the metallized surfaces 4 and 5 are selectively coated with solder.
According to the first example shown in FIG. 10, however, the pressure sensitive adhesive may remain on the surfaces of the electronic component chips 3 when the same are separated from the adhesive face 2 upon completion of the solder coating step. Consequently, two or more electronic component chips 3 may stick to each other, to hinder handling in a later step. Further, since the adhesive forming the adhesive face 2 is not much sufficiently heat resistant, metals employable for coating are restricted so far as the electronic component chips 3 are dipped in the molten bath of a metal.
FIG. 11 shows a second example of an electronic component chip holder, which is to be employed in the plant of the assignee. This electronic component chip holder 6 is entirely in the form of a flat plate, which is provided with holding portions 7 defined by through holes perpendicularly passing through the same. The holding portions 7 are distributed to form rows and columns along the plane of the holder 6. FIG. 12 is an enlarged sectional view showing one of the holding portions 7.
It is understood from FIG. 12 that an elastic member 8 of silicone rubber, for example, is formed on an inner peripheral surface defining the holding portion 7. An electronic component chip 3 is inserted into the holding portion 7 along its longitudinal direction, to be elastically held by the elastic member 7. Then, one metallized surface 4 of the electronic component chip 3 is first exposed from the holder 6.
In order to coat the metallized surfaces 4 and 5 with solder, for example, the holder 6 is turned over from the state shown in FIG. 12 to dip the electronic component chip 3 in a molten bath of solder while holding the same, so as to bring the metallized surface 4 into contact with the molten bath. Thereafter, the electronic component chip 3 is raised up from the molten bath of solder. Thus, the metallized surface 4 is coated with solder. Then, the electronic component chip 3 is moved within the holding portion 7 along its longitudinal direction, so that the other metallized surface 5 is exposed from the holder 6. The electronic component chip 3 is then dipped in the molten bath of solder to bring the metallized surface 5 into contact with the molten metal, and thereafter is raised up from the same. Thus, both of the metallized surfaces 4 and 5 of the electronic component chip 3 are coated with solder.
In the aforementioned second example, however, the metallized surfaces 4 and 5 are separately coated with solder so as to require two soldering steps, which is undesirable in view of productivity.
Further, when the electronic component chip 3 is moved within the holding portion 7 after the metallized surface 4 is coated with solder to expose the other metallized surface 5 as shown in FIG. 13, for example, a solder film 9 formed on the metallized surface 4 may be scraped and stained by the elastic member 8, to deteriorate the appearance of the electronic component chip 3.
Further, since the metallized surface 4 is coated with solder in the formation of the solder film 9 as shown in FIG. 13, the solder film 9 may be remelted by heat upon dipping of the metallized surface 5 in the molten bath of solder.
In addition, air may remain between the lower surface of the holder 6 and the liquid surface of the molten bath of solder when the metallized surface 5 is coated with solder as shown in FIG. 13, for example, whereby an air bubble 10, shown by a broken line, may be formed on the lower surface of the holder 6. Such an air bubble 10 prevents the metallized surface 5 from proper contact with the solder, to cause a defect in the solder film to be formed on the metallized surface 5.
When the electronic component chip 3 is raised up from the molten bath of solder after the metallized surface 5 is brought into contact with the molten bath of solder, a solder drop 11 may be formed as shown by a two-dot chain line in FIG. 13. Such a solder drop 11 arises as the downwardly directed area of the metallized surface 5 is increased when the same is raised up from the molten bath of solder. Thus, upward movement of the electronic component chip 3 in the direction shown in FIG. 13 leads to the largest volume of such a solder drop 11. When the solder drop 11 is solidified in such a state, the appearance of the electronic component chip 3 is deteriorated while the volume of solder used therefor is increased, leading to an undesirable increase in cost. The above description with reference to the metallized surface 5 also applies to the metallized surface 4.